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• MYSQL and the LIMIT clause

  - by Lizard

  I was wondering if adding a LIMIT 1 to a query would speed up the processing? For example... I have a query that will most of the time return 1 result, but will occasionaly return 10's, 100's or even 1000's of records. But I will only ever want the first record. Would the limit 1 speed things up or make no difference? I know I could use GROUP BY to return 1 result but that would just add more computation. Any thoughts gladly accepted! Thanks

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• How do I get callgrind to dump source line information?

  - by Jeremybub

  I'm trying to profile a shared library on GNU/Linux which does real-time audio processing, so performance is important. I run another program which hooks it up to the audio input and output of my system, and profile that with callgrind. Looking at the results in KCacheGrind, I get great information about what functions are taking up most of my time. However, it won't let me look at the line by line information, and instead says I need to compile it with debugging symbols and run the profiling again. The program which I am profiling is not compiled with debug symbols, but the library is. And I know this, because interestingly, source code annotations for cachegrind work fine. When I run callgrind, it says the default is to dump source line information, but it just isn't doing that. Is there some way I could force it to, or figure out what's stopping it?

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• How does Task Parallel Library scale on a terminal server or in a web application?

  - by Lasse V. Karlsen

  I understand that the TPL uses work-stealing queues for its tasks when I execute things like Parallel.For and similar constructs. If I understand this correctly, the construct will spin up a number of tasks, where each will start processing items. If one of the tasks complete their allotted items, it will start stealing items from the other tasks which hasn't yet completed theirs. This solves the problem where items 1-100 are cheap to process and items 101-200 are costly, and one of the two tasks would just sit idle until the other completed. (I know this is a simplified exaplanation.) However, how will this scale on a terminal server or in a web application (assuming we use TPL in code that would run in the web app)? Can we risk saturating the CPUs with tasks just because there are N instances of our application running side by side? Is there any information on this topic that I should read? I've yet to find anything in particular, but that doesn't mean there is none.

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• performance of parameterized queries for different db's

  - by tuinstoel

  A lot of people know that it is important to use parameterized queries to prevent sql injection attacks. Parameterized queries are also much faster in sqlite and oracle when doing online transaction processing because the query optimizer doesn't have to reparse every parameterized sql statement before executing. I've seen sqlite becoming 3 times faster when you use parameterized queries, oracle can become 10 times faster when you use parameterized queries in some extreme cases with a lot of concurrency. How about other db's like mysql, ms sql, db2 and postgresql? Is there an equal difference in performance between parameterized queries and literal queries?

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• help with matlab and Discrete Fourier transform

  - by user504363

  Hi all I have previous experience with Matlab, but the problem that I face some problems in apply a problem in (DSP : Digital signal processing) which is not my study field, but I must finish that problems in days to complete my project. all i want is help me with method and steps of solving this problem in matlab and then I can write the code with myself. the problem is about the signal x(t) = exp(-a*t); 1) what's the Discrete Fourier transform of the sampled signal with sample rate fs 2) if a=1 and fs =1 , plot the amplitude spectrum of sampled signal 3) fix the sampling frequency at fs = 1(hz) [what's it mean ?] and plot the magnitude of the Fourier Transform of the sampled signal at various values of a thanks

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• Problem w/ Paperclip, MacPorts, ImageMagick & Snow Leopard

  - by Kyle Decot

  I'm attempting to use ImageMagick along w/ Paperclip to handle the images on my rails app. The problem is whenever I try to upload an image I get the following in the terminal: [paperclip] An error was received while processing: #<Paperclip::NotIdentifiedByImageMagickError: /var/folders/go/goZ833AaFaqyvv5RnLqQmE+++TM/-Tmp-/stream20110107-6356-1xfs9j1-0.jpg is not recognized by the 'identify' command.> I have added the following to my environments/development.rb file: Paperclip.options[:command_path] = "/usr/local/bin" If I try to interact w/ ImageMagick in the terminal by using "convert" or something similar I get: dyld: Library not loaded: /opt/local/lib/libltdl.7.dylib Referenced from: /usr/local/bin/convert Reason: Incompatible library version: convert requires version 10.0.0 or later, but libltdl.7.dylib provides version 9.0.0 Trace/BPT trap I've already tried updating everything w/ port but the problem still persists. Does anyone have any ideas or suggestions?

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• Memory Warning but Small Live Bytes

  - by Kamchatka

  Hi everyone, In my application, I get a memory warning of level 1 and then 2 after repeating some action (choosing a picture + processing) several times and then a crash. The leak tool doesn't show any leak. I'm also following the Allocations tool in Instruments and my Live Bytes are roughly 4 MB, overall I allocate 113 MB. At maximum I have maybe 20 MB in memory when the picture is loaded. Since I have to repeat an action to get to the crash, it is very likely to be a memory leak. However, I don't know how to locate it since my live bytes are 4 MB and things supposed to be allocated (apart a small leak of ~100 KB in the UIImagePickerController). How much can I trust the memory leak/allocation tools? Would you have an advice to help me locate the reason of the problem?

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• Struts/JSP/J2EE performance and memory profiling and issues

  - by Berlin Brown

  We are using Struts and having performance issues. And making heavy use of jsp includes, tiles, EL expressions. I am sure this is eating up a lot of memory and processing time. What are some approaches to profile the JSP page? What tools could I use? What should I look for when profiling? I have seen the code generated JSP Java Servlet Code and I see the bottlenecks but would rather measure it more accurately. This is under JDK1.5 and IBM Websphere 6.1 (RAD7)

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• Best way to return a user-generated file, AJAX or Forms?

  - by Bill Zimmerman

  Hi, I'm new to web programming, so I need some help. I am writing a custom file-creation app for my site. A user visits the page, clicks on some various options and toggles some checkboxes, and the presses a 'download now' link. I have a PHP backend which will be processing the submission, and generating a PDF file. After the user presses the download link, I want the download to start like it would for any static link. My question is: What is the best way to do this? From my limited understanding, I have a choice between using AJAX or somehow using forms to submit the data. What are the advantages/disadvantages of each? Does anyone have any good links to examples? Thanks

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• How to block non-browser clients from submitting a request?

  - by Thomas Kohl

  I want to block non-browser clients from accessing certain pages / successfully making a request. The website content is served to authenticated users. What happens is that our user gives his credentials to our website to 3rd party - it can be another website or a mobile application - that performs requests on his behalf. Say there is a form that the user fills out and sends a message. Can I protect this form so that the server processing the submission can tell whether the user has submitted it directly from the browser or not? I don't want to use CAPTCHA for usability reasons. Can I do it with some javascript?

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• Rails messing up with HTTP POST Params

  - by Julien Genestoux

  Our app provides an API that people can use to submit URLs like this: curl -X POST http://app.local/resource -d'url=http://news.google.com/newshl=en&q=obama&um=1&ie=UTF-8&output=rss' Unfortunately, it seems that Rails messes up with this param. Any idea on how to fix this? See the log below : Processing ApplicationController#index (for 127.0.0.1 at 2010-06-08 19:03:09) [POST] Parameters: {"um"=>"1", "url"=>"http://news.google.com/newshl=en", "output"=>"rss", "q"=>"obama", "ie"=>"UTF-8"} I would expect the following : Parameters: {"url"=>"hhttp://news.google.com/newshl=en&q=obama&um=1&ie=UTF-8&output=rss"}

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• XSD - problem with schema generation

  - by lm

  Hello I'm tring to generate schema for some type from assembly with xsd.exe here is command line : xsd.exe TestAssemby.dll /t:TestType Here is error I got Error: There was an error processing 'TestAssemby.dll'. - Unable to load one or more of the requested types. Retrieve the LoaderExceptions property for more information I copied referenced dll into folder where is located according to http://social.msdn.microsoft.com/Forums/en/xmlandnetfx/thread/308e4f3a-e1c3-43f5-b7a6-a82f1395997d but still get the same error I have questions : 1) What can be possible reasons of such error ? 2) How should be used LoaderExceptions property ? ( example will be very helpful) Thanks in advance

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• Quick Question: How can I make sure the current directory is not changed because of OpenFileDialog?

  - by pecker

  say my application is installed in C:\programfiles\xyz\ I've some setting & other data files (*.dat files) in this directory too. My application uses OpenFileDialog to open an image. Problem is that when ever user browses to some directory (say MyPictures) for an image. The current working directory of the application becomes that directory (MyPictures in this case). After user inputs the image. I do some processing over it and save some values to imagedata.dat which will be located in the path where original application is installed.(C:\programfiles\xyz here ) In my code I'm just using FileStream("imagedata.dat",...,...) without any path prefix before the filename. Problem here is that application is crashing because it is searching for imagedata.dat in 'MyPictures\imagedata.dat'. How can I avoid this?

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• How do I hide data/field in a List in J2ME

  - by pi

  Hi, I am working on a J2ME app which has dynamically generated Lists. Items on this list may be consequently selected, and the selection processed within the commandAction block. Is there a way to have the IDs of variables populating the List (from a remote database) included in the List item definition as in: this.append("A", null); this.append("B", null); or: String[] arrayOfValues = {"A", "B"}; new List("Menu", List.IMPLICIT, arrayOfValues, null); such that when an item is selected, perhaps A, I can also have its database ID for perhaps further processing. Is it possible to hide a field/data? Thanks.

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• Can Qt's QWebView display programatically generated XML?

  - by Dan Ellis

  Using Qt 4.6, I can dynamically add to an HTML page like this: ui->webView->page()->mainFrame()->documentElement().findFirst("body").appendInside("<i>some text</i>\n"); However, if I QWebView::load() an XML file, it displays it with the correct CSS styling (from an processing instruction), but doesn't show any changes when I do something like: ui->webView->page()->mainFrame()->documentElement().findFirst("lines").appendInside("<line>hello</line>\n"); Why is it treating them differently, and is there a way to modify an XML document in the same way I can an HTML one?

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• Is there a way to rollback and exit a psql script on error?

  - by metanaito

  I have a psql script that looks like this: -- first set of statements begin sql statement; sql statement; sql statement; exception when others then rollback write some output (here I want to exit the entire script and not continue on to the next set of statements) end / -- another set of statements begin sql statement; sql statement; sql statement; exception when others then rollback write some output (here I want to exit the entire script and not continue) end / ... and so on Is it possible to exit the script and stop processing the rest of the script?

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• Rails ActiveRecord: Find All Users Except Current User

  - by SingleShot

  I feel this should be very simple but my brain is short-circuiting on it. If I have an object representing the current user, and want to query for all users except the current user, how can I do this, taking into account that the current user can sometimes be nil? This is what I am doing right now: def index @users = User.all @users.delete current_user end What I don't like is that I am doing post-processing on the query result. Besides feeling a little wrong, I don't think this will work nicely if I convert the query over to be run with will_paginate. Any suggestions for how to do this with a query? Thanks.

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• How to verify a digital signature with openssl

  - by Aaron Carlino

  I'm using a thirdparty credit card processing service (Paybox) that, after a successful transaction, redirects back to the website with a signature in the URL as a security measure to prevent people from manipulating data. It's supposed to prove that the request originated from this service. So my success URL looks something like this: /success.php?signature=[HUGE HASH] I have no idea where to start with verifying this signature. This service does provide a public key, and I assume I need to create a private key, but I don't know much beyond that. I'm pretty good with linux, and I know I'll have to run some openssl commands. I'm writing the verification script in PHP, which also has native openssl() functions. If anyone could please push me in the right direction with some pseudo code, or even functional code, I'd be very grateful. Thanks.

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• Will there be IQueryable-like additions to IObservable? (.NET Rx)

  - by Jason

  The new IObservable/IObserver frameworks in the System.Reactive library coming in .NET 4.0 are very exciting (see this and this link). It may be too early to speculate, but will there also be a (for lack of a better term) IQueryable-like framework built for these new interfaces as well? One particular use case would be to assist in pre-processing events at the source, rather than in the chain of the receiving calls. For example, if you have a very 'chatty' event interface, using the Subscribe().Where(...) will receive all events through the pipeline and the client does the filtering. What I am wondering is if there will be something akin to IQueryableObservable, whereby these LINQ methods will be 'compiled' into some 'smart' Subscribe implementation in a source. I can imagine certain network server architectures that could use such a framework. Or how about an add-on to SQL Server (or any RDBMS for that matter) that would allow .NET code to receive new data notifications (triggers in code) and would need those notifications filtered server-side.

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• C# Class Instantiation Overflow

  - by Goober

  Scenario I have a C# Win Forms App, where the Main Form contains a loop that creates 3000 Instances of another form (Form B). Inside Form B there are a large number of properties and fields and a bunch of methods that do a fair amount of processing. Question Will the creation of 3000 of these classes give me problems? I'm thinking along the lines of memory exceptions? I've had 1 or 2 already, and I've also had an exception that says something along the lines of "Something went bang and this is usually a sign of corrupt memory somewhere else". Setup I'm using a DevExpress Ribbon Form and I haven't implemented Dispose on anything....Do I need to? Help greatly appreciated.

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• Speeding up jQuery empty() or replaceWith() Functions When Dealing with Large DOM Elements

  - by Levi Hackwith

  Let me start off by apologizing for not giving a code snippet. The project I'm working on is proprietary and I'm afraid I can't show exactly what I'm working on. However, I'll do my best to be descriptive. Here's a breakdown of what goes on in my application: User clicks a button Server retrieves a list of images in the form of a data-table Each row in the table contains 8 data-cells that in turn each contain one hyperlink Each request by the user can contain up to 50 rows (I can change this number if need be) That means the table contains upwards of 800 individual DOM elements My analysis shows that jQuery("#dataTable").empty() and jQuery("#dataTable).replaceWith(tableCloneObject) take up 97% of my overall processing time and take on average 4 - 6 seconds to complete. I'm looking for a way to speed up either of the above mentioned jQuery functions when dealing with massive DOM elements that need to be removed / replaced. I hope my explanation helps.

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• Wait for tasks to get completed in threadpool.

  - by Alien01

  Hello I have created a thread pool in C++ which stores all tasks in a queue. Thread pool start n number of threads which takes tasks from queue , process each task and then delete tasks from queue. Now , I want to wait till all tasks get completed. Checking for empty queue for completion of tasks may not work as , task can be given to each thread and queue can be emptied but still the tasks can in processing mode. I am not getting idea how to wait for all the tasks completion.This is a design problem. Any suggestions?

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• How do we achieve "substring-match" under O(n) time?

  - by Pacerier

  I have an assignment that requires reading a huge file of random inputs, for example: Adana Izmir Adnan Menderes Apt Addis Ababa Aden ADIYAMAN ALDAN Amman Marka Intl Airport Adak Island Adelaide Airport ANURADHAPURA Kodiak Apt DALLAS/ADDISON Ardabil ANDREWS AFB etc.. If I specify a search term, the program is supposed to find the lines whereby a substring occurs. For example, if the search term is "uradha", the program is supposed to show ANURADHAPURA. If the search term is "airport", the program is supposed to show Amman Marka Intl Airport, Adelaide Airport A quote from the assignment specs: "You are to program this application taking efficiency into account as though large amounts of data and processing is involved.." I could easily achieve this functionality using a loop but the performance would be O(n). I was thinking of using a trie but it seems to only work if the substring starts from index 0. I was wondering what solutions are there which gives a performance better than O(n)?

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• Virtual and Physical Memory / OutOfMemoryException

  - by user417518

  Hi, I am working on a 64-bit .Net Windows Service application that essentially loads up a bunch of data for processing. While performing data volume testing, we were able to overwhelm the process and it threw an OutOfMemoryException (I do not have any performance statistics on the process when it failed.) I have a hard time believing that the process requested a chunk of memory that would have exceeded the allowable address space for the process since its running on a 64-bit machine. I do know that the process is running on a machine that is consistently in the neighborhood of 80%-90% physical memory usage. My question is: Can the CLR throw an OutOfMemoryException if the machine is critically low on available physical memory even though the process wouldn't exceed it's allowable amount of virtual memory? Thanks for your help!

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• glReadPixels and save to image

  - by Julius Petraška

  I have app, where user drags and drops image, and it is being redrawn with OpenGL for some aviable processing. Everything works. And when user wants to save his image it works like that: glReadPixels -> NSBitmapImageRep -> NSData -> Write to file This works too. Almost. With some images it is not working as it should work. For example: .png when I open and save this image: I get: And if I open and save this image: I get: .jpg If I open and save: I get: And when I open and save: I get: So sometimes images saves badly. Why is it happening?

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